Tetrahydrofuran (THF), a widely used organic solvent and a starting material for the synthesis of poly(tetramethylene ether) glycols, can be produced by many well-known processes.
Catalytic hydrogenation of furan or maleic anhydride gives tetrahydrofuran, as disclosed in U.S. Pat. Nos. 2,772,293 and 3,021,342. Acid-catalyzed cyclodehydration of 1,4-butanediol in the presence of soluble acid catalysts is another well-known method of preparing THF.
A commercially important synthetic route to 1,4-butanediol involves hydroformylation of allyl alcohol in the presence of a rhodium catalyst to give a mixture of 4-hydroxybutanal (HBA) and 3-hydroxy-2-methylpropanal (HMPA). Catalytic hydrogenation of the aldehyde mixture under neutral conditions gives a mixture of 1,4-butanediol (BDO) and 2-methyl-1,3-propanediol (MPD). The 1,4-butanediol can then be cyclodehydrated under acid conditions to produce tetrahydrofuran. Alternatively, THF can be produced directly from 4-hydroxybutanal by hydrogenating the aldehyde mixture under acidic conditions, as disclosed in U.S. Pat. Nos. 4,064,145 and 4,105,678.
U.S. Pat. No. 4,105,678 teaches (Examples VI and VII) that an aqueous mixture of 4-hydroxybutanal and 3-hydroxy-2-methylpropanal can be hydrogenated with palladium on carbon in the presence of acetic acid at 114.degree. C. to give a mixture of 1,4-butanediol, tetrahydrofuran, and 2-methyl-1,3-propanediol. 4-Hydroxybutanal was quantitatively converted to THF using palladium on carbon by increasing the reaction temperature to 190.degree. C.
The processes described in U.S. Pat. Nos. 4,105,678 and 4,064,145 suffer from several disadvantages. The activity of palladium on carbon is insufficient for producing satisfactory yields of both THF and 2-methyl-1,3-propanediol. The use of soluble acids such as acetic acid results in low yields of 2-methyl-1,3-propanediol (less than 3%) because 3-hydroxy-2-methylpropanal dehydrates under the reaction conditions to give 1-propen-2-al (methacrolein), which subsequently polymerizes or is hydrogenated to give isobutyl alcohol. Soluble acids also catalyze undesirable aldol condensation reactions. Product isolation in the presence of soluble acids is difficult, and yield losses through dehydration and esterification reactions are significant. Recovery and reuse of soluble acids is impractical and uneconomical.
Thus, while an acid catalyst is needed to promote cyclodehydration of 4-hydroxybutanal, strongly acidic conditions promote unwanted dehydration of 3-hydroxy-2-methylpropanal, which results in low yields of 2-methyl-1,3-propanediol.
It is therefore an object of this invention to provide a process for producing tetrahydrofuran from 4-hydroxybutanal using a supported dehydration/hydrogenation catalyst to overcome the problems of soluble acid catalysts. It is also an object of the invention to improve the activity of the catalyst relative to the palladium-on-carbon system. A further object is to develop a THF process that gives good selectivity in the conversion of 3-hydroxy-2-methylpropanal to 2-methyl-1,3-propanediol.